1. Field of the Invention
The present invention relates to a numerical control apparatus (hereinafter, “NC apparatus”) and a numerical control machine tool (hereinafter, “NC system”), and in particular to an NC apparatus that can control a machine tool having a shaft (hereinafter, “clamp shaft”) capable of performing clamping, and an NC system that includes the NC apparatus.
2. Description of the Related Art
An NC system controls driving of servomotors having a plurality of shafts with a servo control apparatus to control movement of a tool relative to a workpiece, thereby machining the workpiece. The tool needs to be held rigidly while the machining is on its way. The tool is in generally held rigidly by the shafts of the servomotors. However, to enhance the rigidity, a clamp apparatus that can mechanically fix a position of a shaft, such as a coupling of a meshing type, is used. The clamp apparatus is used on a shaft where a disturbance (cutting resistance) is large relative to a torque capacity of a servomotor. Thus, the clamp apparatus is generally used on a rotary shaft for a turret tool rest or for a turntable.
Although the clamp apparatus enhances the rigidity, its presence can cause a positioning accuracy of the shaft to degrade. When the clamp apparatus is used on a shaft, and if the clamp apparatus is operating (hereafter, “clamping state”), a positional accuracy of the shaft depends mainly on a mechanical precision of the clamp apparatus (for example, precision of a coupling). On the other hand, if the clamp apparatus is not operating (hereafter, “non-clamping state”), the positional accuracy of the shaft depends mainly on a positioning accuracy of the servo motor. As a result, a difference in the position of the shaft occurs between the clamping state and the non-clamping state. If the shaft is positioned with the servomotor in a non-clamping state, and if the shaft is clamped with a clamp apparatus, problems such as generation of noises due to a meshing error or hitting of teeth to each other can occur.
A typical NC lathe can turn a tool rest having a plurality of tool holding faces to determine a desired tool holding face to a predetermined angle. In such an NC lathe, an angle of a motor actually detected in a state that the tool rest has been clamped at a predetermined angle, or an angle error to a reference value is stored, and an angle target value of the motor is corrected, when the tool rest is next positioned at the same angle in the non-clamping state. Accordingly, an angle deviation that occurs when the tool rest is switched to the non-clamping state to the clamping state can be solved. A conventional NC lathe is disclosed in, for example, Japanese Patent Application Laid-open No. 2000-5978 (P. 3 to 5, and Fig. 1).
In the conventional technique, however, only a positional error (angular error) in a moving direction of the clamp shaft (a pivot shaft of the tool rest) that occurs due to a difference between the clamping state and the non-clamping state is set as an object to be corrected. In fact, although very small, positional errors occur even in other directions (for example, an axial direction of the pivot shaft) due to the difference between the clamping state and the non-clamping state. In the conventional technique, the positional error in the other moving directions can not be corrected.
In the conventional technique, a correction for the positional error (the angular error) is performed only in the non-clamping state, and no correction for the positional error is performed in the clamping state. In such a machining process in which machining in the clamping state and machining in the non-clamping state are performed, it is necessary to control the position of a tool with a high accuracy in both of the clamping state and the non-clamping state. In the conventional technique, however, there is a problem that the position of the tool cannot be controlled with a high accuracy neither in the clamping state nor in the non-clamping state.